Generally, the term stent is a commonly used medical term to refer to a cannula or cylindrical- or tubular-shaped device for endoluminal, usually endovascular, use which is placed inside an anatomical structure or a duct in the body in order to keep it permeable or to prevent its collapse after surgical release, dilation, or clearance. A stent is usually implanted in a blood vessel at the site of an endoluminal aneurysm or stenosis, i.e., by means of so-called “minimally invasive techniques”, in which the stent is contained in a configuration that is radially compressed by a tube or catheter and is delivered by means of a stent applicator or “introducer” in required location. The introducer can enter the body from an access site outside said body, such as through the skin of the patient, or by means of a sectioning technique in which the incoming blood vessel is exposed to minor surgical procedures.
As it is used herein, the term stent also refers to grafts, stent grafts, vein cava filters, expanding structures and similar implantable medical devices, which are radially expanding endoprotheses. They are normally are intravascular implants that can be transluminally implanted and they radially enlarge after having been introduced percutaneously.
Stents can be implanted in various cavities or vessels in the body, such as in the vascular system, the urinary tract, bile ducts, among others. Said stents can be used to reinforce blood vessels and to prevent re-stenosis followed by angioplasty in the vascular system. Stents can be self-expanding, such as nitinol shape memory stents; furthermore, mechanically expanding, such as an expanding balloon stent; or expanding hybrid stents.
The use of endoluminal stents is very common in different areas of medicine and veterinary science. There are different stent designs for endoluminal insertion in blood vessels and other lumina to prevent or reverse the blocking thereof.
It is generally considered that there are three basic categories of stent type devices, namely:                heat-expandable devices,        expanding balloon devices, and        self-expanding devices.        
Self-expanding type stent devices, which can optionally be heat-expandable, are inserted in a vessel in the body in radially compressed form and mechanically transition to a radially expanded position. Once the stent is placed in the desired position in the blood vessel, it expands radially, applying outward pressure on the inner surface of the wall of the vessel in the body in which it has been placed.
In turn, there are braided stents and unbraided stents. Braided stents are made by braiding (interweaving) wires of a fine metallic material according to different braiding patterns. Patent document U.S. Pat. No. 6,083,257A discloses a methodology for braiding stents. Depending on the number of wires, the braiding angle, the nominal radius, the nominal length and the braiding pattern used, the mechanical properties and density of the resulting stent mesh can vary considerably.
Stents are often used for treating intracranial aneurysms (IA), a sector where there are different types of braided stents. One of those types is known as a “flow diverter” (FD), which is densely braided and placed longitudinally along the vessel affected by the aneurysm and covers the neck of the aneurysm. Alternatively, thickly braided stents are also used as a scaffold for protecting the neck of the IA after placing an endovascular coil, as disclosed in patent document U.S. Pat. No. 6,010,468A.
Stents are placed in the desired site by means of a catheter in image-guided operations, conventionally with X-ray image guiding, the interventionalist with the aid of a contrast agent that highlights the location of the lumen of the vessel and, where appropriate, of the aneurysm to be treated. In the case of aneurysms, the catheter is inserted in the body normally through arteries, for example the iliac artery, and is led to the location of the aneurysm by a neurointerventional radiologist. Said radiologist will select the position in which the distal side of the stent is placed and will progressively take the stent out of its sheathe until it is completely released in the treated vessel.
Stents have the difficulty that the final porosity of the stent is not known a priori when it is placed inside the body and the value of which determines both the amount of flow entering the aneurysm to be treated and the vessels adjacent that are covered by the stent.
Porosity of a stent when it is placed inside a vessel can be approximated assuming that the stent is released in a straight vessel having a constant radius. This calculation consists of determining the area of the outer wall of the cylinder generating the stent, based on its radius and length, and the surface area of metal covering said cylinder, based on the number of wires, the thickness of each wire, its length and the number of cross points between wires on the surface of the stent. This method provides rather imprecise approximations of the porosity the stent will have once it is inserted in the patient's vasculature given that vessels are generally heterogeneous tubular structures both in terms of radius and in terms of their three-dimensional morphology, having curvatures and twisting.
When a stent is located outside of a structure delimiting it, such as a vessel, as previously mentioned, it adopts its maximum radius and minimum length in the absence of stresses. However, if said stent is placed inside a vessel having a radius smaller than the radius it has outside a structure delimiting it, the walls of the vessel delimit the radial expansion of the stent, forcing the device to expand longitudinally until achieving a balance situation. This makes the stent in the vessel have a longer length than it does in the air. This, added to the fact that it is deployed in a curved tubular structure, causes porosity to depend on the point of the surface of the stent on which said porosity is measured. Therefore, measuring porosity of said device before placement thereof does not provide realistic values about the behavior of the stent once it is inserted. The interventionalist does not have tools for estimating, a priori, porosity of the stent once it is placed inside the patient. In the case of intracranial aneurysms, the variation in the density of the stent mesh as a result of the different degrees of expansion and curvature to which the stent is subjected means that the effect of the device on blood flow inside the aneurysm is difficult to predict. For this reason, there is a need to provide a tool that allows precisely predicting the final porosity of a stent once it is placed inside the body.
There are background documents describing methods for modeling stents. Deformable models have been used to simulate the behavior of a stent when it is placed inside the lumen of a vessel (Larrabide, I. et al. “Fast virtual deployment of self-expandable stents: method and in vitro evaluation for intracranial aneurysmal stenting.” Medical image analysis, 2012, 16(3), 721-730). However, said method does not allow predicting porosity of the stent, given that it does not take into account its longitudinal deformation.
Other methods based on the mechanical deformation of a cylinder-like structure have also been proposed (Cebral, J. R. and Lohner, R. “Efficient simulation of blood flow past complex endovascular devices using adaptive embedding technique.” IEEE Transactions on Medical Imaging, 2005, 24(4), 468-476), but they are not able to predict the change in porosity of the stent either.
A method based on the use of finite elements and a detailed description of the braiding pattern, which allows a more precise modeling of the mechanical behavior of the stent type device, has recently been disclosed (Ma, D. et al. “Computer modelling of deployment and mechanical expansion of neurovascular flow diverter in patient-specific intracranial aneurysms.” Journal of biomechanics, 2012, 1-8). This method is considerably precise when modeling the behavior of a stent, but obtaining the models is extremely complex and computationally expensive.
Other methods based on obtaining images of the lumina of the vessels to be treated and modeling for determining the most suitable stent are those disclosed in international patent applications WO2006/093776 and WO2011/038044, and in US patent application US2007/0135707, although none of them describes use thereof for determining the porosity of the stent.
International patent application WO2006/093776 discloses a method of modeling stents based on using an ultrasonic imaging system for obtaining images of blood vessels, detecting defects in said vessels and using said images to perform graphic simulations with different stents to check if the length and position are suitable.
International patent application WO2011/038044, in turn, discloses an automated method for simulating the length and position of stents based on obtaining images of the lumen of the blood vessel by means of optical coherence tomography. A three-dimensional reconstruction of the contours of the lumen of the vessel is performed from the images obtained, data relating to the diameter of the vessel and to the blood flow rate, pressure and resistance is obtained to finally simulate and optimize the length and/or position of the stent.
Finally, US patent application US2007/0135707 discloses obtaining three-dimensional images used to build a model of the vessel to be treated to detect the lesion and its characteristics and to simulate the stent to be used and the position in which it will be placed.
The authors of the present invention do not know of any method or system that allows determining porosity of a stent or of any other class of flexible porous structure, whether tubular or not, when it is subjected to deformation, by means of processing representative data of the flexible porous structure, i.e., without being able to directly check on the flexible porous structure the porosity that it has, for example, since the porous structure is placed in an inaccessible site, as is the case of a stent implanted inside the human body.
There is a need, therefore, to provide a solution to the objective technical problem relating to how to determine porosity of a flexible porous structure when it is subjected to deformation by means of processing representative data thereof.